1. Field of the Invention
The present invention relates to a balun that performs unbalanced-to-balanced signal conversion, and more particularly to a thin film balun formed by a thin film process which is advantageous for miniaturization and thickness reduction.
2. Description of the Related Art
Radio communication apparatuses include various high frequency elements such as antenna, filter, RF switch, power amplifier, RF-IC and balun. Here, resonant elements such as antenna and filter handle unbalanced signals relative to the ground potential; however, RF-IC, which produces or processes high frequency signals, handles balanced signals. Thus, in connecting the two types of elements, a balun is used which functions as an unbalanced-to-balanced signal converter.
Sometimes a balanced signal needs to be amplified by applying a bias to a balun used in mobile communication equipment such as mobile-phone and wireless LAN. In this case, between the balanced transmission line of the balun and the ground terminal, a capacitor must be connected in series which has a capacitance which allows the intended frequency signal to flow to GND, so that the DC component and the intended frequency component are separated.
As a balun having such function, Japanese Patent No. 3780414 discloses a laminated balun which has a capacitor disposed between a balanced transmission line and GND terminal of the balun. This laminated balun is formed by using LTCC (Low Temperature Co-fired Ceramics) technique, and the capacitor is formed over the whole area.
To meet the recent demands for miniaturization and thickness reduction of electronic devices, the present inventor has made a diligent study on characteristics of a balun including the related art bias supply capacitor. As a result, it was found that, in a thin film balun formed by a thin film process, the related art bias supply capacitor affects balanced transmission characteristics of the balun. It is expected that this is caused by a structural factor that thin film baluns have a short interlaminar distance between the capacitor and balanced transmission line, compared to laminated baluns. However, it became clear that, as long as the structure of the related art laminated balun is used, various characteristics required by thin film baluns cannot be further improved to a satisfactory level.
To address the above problem, the present invention has been devised, and its object is to provide a thin film balun which, while incorporating a capacitor used to supply a DC bias, allows improvement in balanced transmission characteristics.